The reason for them to refuse when no negative factors are involved is unknown. It happens, and it might be that they may soften up after getting to know you better (i.e. you have a better standing with the village), although that's speculation. It may also be that they're less likely to accept jobs during sowing and harvest seasons, although that's speculation as well.
There was a bug that caused former hirelings to never get out of "not now, maybe later", but I believe that bug has been fixed.
However, if they say they're not interested I think you're out of luck for quite some time at the least (and they're in luck for not getting murdered by a greedy serial killer).